1. FIELD OF THE INVENTION
The present invention relates to pellets of copolymers of a lower olefin and an ethylenically unsaturated carboxylic acid which have been surface treated with a low level of finely divided silicone dioxide. The thus treated pellets exhibit dramatically increased tack resistance as measured by the stick temperature test.
2. PRIOR ART
U.S. Pat. No. 3,909,487 discloses the treatment of fine particles of ethylene/ethylenically unsaturated carboxylic acid copolymers or corresponding ionomers with silane-treated fumed silica. The polymers are rough, spherical particles of 100 mesh or finer, preferably 10-100 .mu.m diameter. 0.1-0.5% by weight of hydrophobic aliphatic silane treated silicone dioxide is added to the polymer. This amount of silica is rather high on a wt % basis, compared to the current invention, but rather low on a g/m.sup.2 basis. The adhesion of the composition to glass is improved by the presence of the silica. There is no disclosure of reduced tackiness of the polymer.
U.S. Pat. No. 3,595,827 discloses a composition of acid-containing copolymers with 0.02-2.0% amide wax ("amphiphatic compound") and 0.01-2% synthetic amorphous silica. The silica is not used as a surface treatment. It may be added by melt extrusion, for example, and is not dusted onto the polymer. It is disclosed that the silica particles provide channels through the bulk of the material which permits the wax to rapidly migrate to the surface. The patent states that use of silica alone (without the wax) "has at best an insignificant effect." Only amorphous silica can be used--many other types do not work.
U.S. Pat. No. 3,463,751 discloses the use of a wide variety of antisticking agents (including silica) in E/VA copolymers which may contain up to 20% methacrylic acid. The actual antisticking agent is a mixture of PVC powder with a second agent, such as silica. "It is essential to use the polyvinyl chloride powder together with the antisticking agent. If the use of either of them is omitted, there is scarcely effective for the purpose of the present invention." The copolymer which is treated is in the form of a disperison, in contrast to the present invention, which is preferable for the treatment of pellets.